Voice switched speakerphone with filters having exponential transfer function in the monitoring path

ABSTRACT

A handsfree telephone circuit has separate transmit and receive audio paths for connection to a microphone and speaker respectively. The circuit comprises a controllable attenuator in each of said transmit and receive paths, an attenuator control responsive to control signals to set the attenuation level of said attenuators, and a separate monitoring circuit connected to each of said transmit and receive paths for sensing audio signals therein. Each monitoring circuit comprises a filter for band limiting the audio signals, a peak level detector receiving said band limited signals, and a speech detector for distinguishing speech from background noise. A switching comparator compares the outputs of the peak level detectors in the two monitoring circuits and produces directional control signals for said attenuator control means to determine which audio path has control. The circuit eliminates &#34;hollow&#34; effect in the speech due to bandlimiting filtering in the audio paths.

This invention relates to telephones, and more particularly to ahandsfree circuit for use in a telephone set.

BACKGROUND OF THE INVENTION

A conventional telephone hand set operates in the full duplex mode,which means that the parties can transmit and receive at the same time.This is possible because a conventional hand set is designed to haveminimal acoustic coupling between the speaker and microphone. Ahandsfree system, which allows the user to operate the telephone withouta hand set, has difficultly operating in the full duplex mode because ofthe high degree of acoustic coupling between the microphone and loudspeaker. Most handsfree systems are therefore operated in the halfduplex mode, which means that some form of switching must be provided inthe transmit and receive paths.

In a conventional handsfree circuit, variable complementary lossattenuators are inserted in the transmit and receive paths. These arecontrolled in accordance with the audio signals in the two paths. Leveldetectors determine the overall noise level in the respective paths andspeech detectors distinguish between background noise, which is assumedto be a generally constant level, and speech, which is generallycharacterized by short bursts of higher level audio. One example of sucha system is described in U.S. Pat. No. 4,490,582.

Prior art designs have generally required filters in the transmit and/orreceive audio paths to band limit the speech path to the 400 to 3500 Hzregion. These filters are required to reduce background noise otherheavy machinery, but they create a "hollow" effect that degrades thequality of the transmitted sound.

A further problem with the prior art circuits stems from the fact thatthe speech detectors work by detecting short bursts of sound.Consequently, many prior art circuits will respond to intermittentbackground noise, such as typing, causing the attenuators to ramp up tothe full transmit state when no speech is present.

Yet another problem with the prior art design is the frequent presenceof echo signals due to trans-hybrid reflection or reverberation in theroom. Peak detector decay rate is made long so as to preventreverberation echo from causing the handsfree to switch states.

An object of the present invention is to alleviate the aforementionedproblems with the prior art.

SUMMARY OF THE INVENTION

According to the present invention there is provided a handsfreetelephone circuit with separate transmit and receive audio paths forconnection to a microphone and speaker respectively, comprising acontrollable attenuator in each of said transmit and receive paths;attenuator control means responsive to control signals to set theattenuation level of said attenuators; a separate monitoring circuitdefining respective monitoring paths connected to each of said transmitand receive paths for sensing audio signals therein, each saidmonitoring circuit comprising a filter in the associated monitoring pathfor band limiting said audio signals, a rectifier, a peak level detectorreceiving said band limited signals, and a speech detector fordistinguishing speech from background noise; and a switching comparatorfor comparing the outputs of the peak level detectors in the twomonitoring circuits and producing directional control signals for saidattenuator control means to determine which audio path has controlthereof.

Preferably, a fast attack filter with a slow decay rate is provided ineach of the input paths of the switching comparator. The filter in thenon-controlling path can be bypassed so that the tendency of theswitching comparator to switch on echo signals in the non-controllingpath is reduced because the envelope from the filter in the controllingpath extends beyond the echo signals in the non-controlling path.

The filter in the audio paths is preferably a second order biquadfilter. The general 2^(nd) order Z-domain transfer function is asfollows:

    Y.sub.(z) /X.sub.(z) =G(1+A.sub.1 Z.sup.-1 +A.sub.2 Z.sup.-2)/(1-B.sub.1 Z.sup.-1 -B.sub.2 Z.sup.-2)

where G is the gain, A₁, A₂, B₁, B₂ are constants that define the filterfunction.

The preferred values of G, A, A₁, A₂, B₁, B₂ are as follows:

G=0.2054

A₁ =-1.9911

A₂ =+1.0000

B₁ =+1.6067

B₂ =-0.67222

The use of the second order biquadratic filter in the control path thatsenses the transmit and audio signals allows the audio paths to use awider band width (200 Hz to 3500 Hz) and thus avoid the "hollow" effectthat band limiting creates in a prior art circuit with a filter directlyin the audio paths.

The attenuators are preferably in the form of digital attenuators havingthree stable states, namely idle, and full transmit and full receivestates, and a number of intermediate transient states through which theattenuators are ramped between the stable states. Each intermediatestate is preferably separated by an attenuation of 1.5 dB, and theattenuators are preferably ramped up between the intermediate states atthe rate of one state every 0.5 milliseconds when going from the idle tothe fully "on" state. The attenuators decay slowly back to the idlestates, taking 1 or 2 seconds in the absence of speech signals. The ramptime in the decay direction is programmable.

The peak detectors follow the envelope of the filter outputs, afterbeing rectified. A fast attack and decay rate is used. If the speechdetectors are triggered momentarily by short deviations in backgroundnoise, such as may be caused by typing at the keyboard, the peakdetector output will decay back to the average noise level faster thanthe attenuators can swing to the full transmit state. Thus, although theattenuators might start to swing on a noise spike, they will return tothe correct state more quickly than prior art circuits that have a longexponential rate of decay for the peak detectors.

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a handsfree telephone circuit in accordancewith the invention;

FIG. 2 is a block diagrammatic representation of a second orderbiquadratic digital high pass filters;

FIG. 3 is a block diagram of a peak detector filter;

FIG. 4 is a block diagram of a speech detector comparator;

FIG. 5 is a block diagram of a noise level detector filter;

FIG. 6 is a block diagram of a detection comparator;

FIG. 7 is a block diagram of a comparator low pass filter;

FIG. 8 is a diagram showing the output wave forms of the audio signals;

FIG. 9 is an attenuator state diagram; and

FIG. 10 is a flow chart showing the attenuator control algorithm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the handsfree circuit comprises a two-to-fourwire hybrid 1 connected to a full duplex telephone line 2 on one sideand on the other side to respective transmit and receive paths 3, 4.

The transmit path 3 includes an amplifier 5 connected to a microphone 6,an analog-to-digital converter 7, and a variable loss attenuator 8. Thereceive path 4 also includes a variable loss attenuator 9, adigital-to-analog converter 10 and an amplifier 11 connected to aspeaker 12.

Each attenuator 8, 9 has sixty-four states separated by 1.5 dB loss. Theattenuators 8, 9 are controlled by an attenuator control unit 13 in acomplementary fashion so that as the loss introduced by one attenuatoris decreased, the loss introduced by the other attenuator is increasedby a corresponding amount. The attenuators 8, 9 have three stablestates, namely an idle, and a full transmit and a full receive statehaving respectively minimum, intermediate and maximum loss. The lossesintroduced by the attenuators in these states can be preset by gaincontrol inputs 14 to the attenuator control unit 13.

The transmit and receive paths, 3, 4 are associated with respectiveseparate monitoring circuits 14, 15 connected upstream of theattenuators 8, 9 to monitor audio signals in the paths 3, 4. Eachmonitoring circuit 14, 15 comprises a high pass filter 16, a rectifier17, a peak level detector 18, and a speech detector 19. The monitoringpaths are also connected to a common direction control comparator 20which determines which audio path has control of the attenuator controlunit 13.

Each high pass filter 16 comprises a second order biquadratic digitalfilter as shown in more detail in FIG. 2. The filter, which has aprogrammable transfer function uses default filter coefficients for ahigh pass filter with a -3 dB point at 400 Hz. This default filterensures that the circuit will work reliably in environments with a largeamount of low frequency background noise. The filter comprises amultiplier 21, adders 22 and 23, further multipliers 26 and linearfilters 24 and 25. The result of the circuit shown in FIG. 2 is toimplement the equation:

    Y/X=G(1+A.sub.1 Z.sup.-1 +A.sub.2 Z.sup.-2)/(1-B.sub.1 Z.sup.-1 -B.sub.2 Z.sup.-2)

where G, A1, A2, B1 and B2 are constants having values as follows:

G: +0.2054 (-12 dB passband)

A1: -1.9911

A2: +1.0000

B1: +1.6067

B2: -0.6722

The values of the gain G are separately programmable for the Rx and Txmonitoring circuits. The default values are for the filter to have anattenuation of 12 dB in the passband for both circuits.

The rectified output of the high pass filter 16 is fed to a peakdetector filter, shown in more detail in FIG. 3. This comprises an adder27 and first order digital filter 28. The peak detector responds quicklyto a rectified input signal |y(n)|, which has been attenuated by 12 dBin the filter 16.

The speech detector 19, shown in more detail in FIG. 4, comprises anoise detector filter 29, an adder 30 and a comparator 31. The noisedetector filter 29, shown in more detail in FIG. 5, comprises an adder32, an adder 33, and a first order digital filter 34. The comparatorproduces a high output only when peak detector output is higher than thelong-term average noise level. The threshold level is programmable.

The direction control comparator is shown in more detail in FIG. 6. Theoutputs of the peak detector filters 18 are fed to respective low passfilters 35, 36, which have a fast attack and slow decay having aninverted exponential form, as shown in FIG. 9 for reasons that will bedescribed in more detail below. The filter 35 in the transmit path Tx isconnected to the non-inverting input comparator 37, and the filter 36 inthe receive path Rx is connected through a variable attenuator 38 to theinverting input of the operation amplifier 37. The low pass filters 35,36 are shown in more detail in FIG. 7. A constant is added at 38 toregister 39 (dec(n)) to produce a linearly increasing value dec(n) whichis subtracted from register 40 complemented with subtractor 41. When areset occurs, register 40, which is shown as Z⁻¹, is loaded with peak(n)and register 39 is cleared to zero. This creates the invertedexponential decay function of comp(n).

Referring again to FIG. 1, the signals on the full duplex line 2 areassumed to be digital. The hybrid 1 provides an interface between thehalf duplex handsfree circuit and full duplex line 2. The hybrid 1 willalso include D/A and A/D converters since the full duplex line 2 isanalog. Incoming signals are directed from the line 2 into the receivepath 4 and outgoing signals are directed from the transmit path 3 intothe line 2. In the case of the Rx signals, they are attenuated by anamount determined by the setting of the attenuator 9 prior todigital-to-analog conversion and amplification before they are appliedto the speaker 12. When the handsfree circuit is in the full receivemode, i.e. with the Rx path in control, the attenuator 9 will be set forminimum loss so as to allow the incoming signals to pass through.

Similarly, the outgoing signals are passed through amplifier 5,analog-to-digital converter 7, and attenuator 8, which operates in amanner complementary to the attenuator 9. When the circuit is in thetransmit mode, i.e. with the transmit path in control, the attenuator 8is set for minimum loss and the attenuator 9 is set for maximum loss. Inthe idle state, when no speech is present, both attenuators are set formidgain.

A full state diagram of the attenuators 8, 9, is shown in FIG. 9. Eachattenuator has 64 states, each separated by 1.5 dB gain/loss. Three ofthe states are stable, namely full transmit, full receive, and idle,when no speech is detected on either path. The stable states areprogrammable in accordance with system requirements. The attenuators arecontrolled in accordance with the flow chart shown in FIG. 10.

The attenuator control unit 13 provides 6 bit gain control output to theT_(x) and R_(x) attenuators 8, 9. These 6 bit controls select one of 64possible gains, on the ramp which are 1.5 dB apart giving a maximumrange of 64 steps of 1.5 dB.

Of the 64 possible attenuator gain states, the speaker-phone will restin only one of the three stable states (Full-Tx, IDLE, or Full-Rx).During a transition from idle to a full state, or full state to fullstate, the attenuators 8, 9 pass through the other states at a fixedrate of 1.5 dB every 0.5 millisecond. A full-state to idle-statetransition is timed by two programmable timers (not shown); the"HOLD-OVER" timer and the "RAMP-DOWN" timer. The "HOLD-OVER" timerallows the attenuators to hold a full state level for a length of timedetermined by the time preset value before starting a slow decay to idlestate, and the ramp down timer allows the slow decay rate to beprogrammed.

The IDLE, MAXTX and MAXRX levels (FIG. 9) are stored in programmableregisters (not shown) which may be programmed by an externalmicroprocessor (not shown). The choice of an IDLE level will affect theoverall loop attenuation of the speaker-phone. User control of thespeaker volume can be adjusted over the full range of the Rx attenuatorby changing the contents of the Rx gain control register. The valuewritten to the Rx gain control register is used to limit the maximumpositive swing of the attenuator (MAXRX). This feature allows thespeaker-phone to operate "closer to full duplex" at low speaker volumesettings because the attenuators will not have to swing so far.

The maximum gain of the Tx speech path may also be programmed over thefull range of the Tx attenuator with the Tx gain control register.(MAXTX). The following are some recommended restrictions for the Txgain,and IDLE registers:

1. The positive gain range of the Tx attenuator should be used foroptimum noise performance in the transmit path.

    0<=TXGAIN<=48 (base 10)

2. The following restriction applies to the Rx gain setting:

    0<=RXGAIN<=63 (base 10)

3. The following restrictions apply to the IDLE state setting:

    (2*IDLE-RXGAIN)>=0

    (2*IDLE-TXGAIN)>=0

The control inputs to the attenuator control unit 13 that cause theattenuators 8, 9 to change state are derived from the monitoringcircuits 14, 15, which set the audio signals in the speech paths 3,4.There are two types of control signal, namely the signals from thespeech detectors 19 that cause the attenuators 8 to ramp rapidly to thefull state, and the direction control signals from comparator thatdetermine which audio path controls the attenuator control unit 13 atany given time.

The sensed audio signals are first fed through the second order highpass filter 16, which is programmable, but which has a default response,which is high pass and a minus 3 dB point at 400 Hz. This frequencyresponse ensures that the speech detector 19 and switching comparator 20work reliably in environments with large amounts of low frequency noise,such as fans and other machinery, as well as with analog trunk hybridsthat reflect low frequency components. The default value for the gain Gin both the Tx and Rx paths, is 12 dB, but the values for G in each ofthe paths can be programmed separately. It is preferable not to have again more than 12 dB because of word length effects inside the filter.

The peak level detectors 18 produce an output signal representing theenvelope of the waveform of the filtered audio signals, as shown in FIG.8. The peak detector registers 28 are updated once every 125microseconds and thereby represent the short term average audio level.The detector responds immediately to a rectified input signal |y(n)|,which has been attenuated by at least 12 dB in the second order filter.The rate of decay is exponential, with the time constant being, forexample, in the order of 2 milliseconds.

The speech detectors 19 compare the peak detector output with the noiselevel tracked by the noise level detector 29 (FIG. 4), which tracks theaverage noise level at the output of the peak detector 18. Thecomparator output is only high, indicating speech present, when the peakdetector is higher than a long term average noise level plus aprogrammable threshold level, which can be used to control thesensitivity of the speaker phone to deviations in background noiselevel.

The noise level filter 29 takes the long term average of the backgroundnoise level of the peak detector filter output. If the peak detectoroutput is lower than the noise level filter, the noise level filterdecays slowly toward the peak detector output, with a time constant ofabout 16 seconds. If the peak detector output is higher than the noisefilter output, the filter output increments linearly with a full scaletime constant of 1,024 seconds. These long time constants are necessaryto ensure that the noise level average is not moved significantly byspeech present at the peak detector output.

The noise level filter shown in FIG. 5 takes the difference betweenpeak(n) and noise(n) and scales the result down by 15 bits. If peak nminus noise n is negative, then the result after scaling down is alwaysminus 1 (since 16 bit twos complement arithmetic is used). If peak nminus noise n is positive, then the result is always 0. This yields thedesired results since the attack time has to be made very slow. The slowattack time constant is accomplished with the second adder 33, whichalways adds 1 over 64. this is done by adding 1 every 64 sample periods.Since the attack time constant is so long, the initial value for thenoise filter output (at time T=0) is set to a large positive level toprevent start up problems.

Direction control is determined by the direction comparator 20, whichcompares the outputs of the peak detectors 18. Two types of hysteresisare built in. First, depending upon the state of the comparator output,one of the two comparator input signals will be low pass filtered, andsecond the relative levels of the peak outputs are varied according tothe state of the Tx and Rx attenuators.

The low pass filter tends to extend the decay time of the peak detectorsignal in order for the comparator to hold on longer to the directionthat it is currently switched to. For example, assuming that the Tx peakdetector output is large enough to switch the comparator to the Txdirection (positive output), this positive output is used to disable thelow pass filter on the Rx peak detector signal and enable the filter onthe Tx peak detector signal. This effect allows the Tx peak detector tofollow the speech envelope closely without requiring a long decay time,while still preventing the direction comparator from switchinginadvertently to the Rx direction on an echo or reflection from ananalog trunk interface. This hysteresis is even more important when thecomparator switches to the Rx direction. Because of acousticreverberation, the Rx signal that is sent to the speaker will appear inthe Tx path at a much higher level and delayed in time. The slow decaytime of the low pass filter and the effect of switching the low passfilters on and off, prevents acoustic reverberation of Rx speech fromswitching the comparator to the Tx direction.

The secondary hysteresis effect is caused by the variable attenuator 38in the Rx control path (shown in FIG. 6). Depending upon the state ofthe attenuator in the Rx speech path, the maximum level of areverberation signal in the Tx path can be deduced. For example when theRx attenuator is in full receive mode (maximum volume) the attenuator inthe Rx control path (to the -ve input of the direction comparator) isalso at a maximum volume level which is identical to the Rx attenuator.Conversely when the Tx attenuator is in full Tx mode, the Rx attenuatoris at a minimum level and so is the attenuator in the Rx control path.This produces a hysteresis effect on the comparator which makes itgradually harder for the opposing speech path to take control dependingupon how far the attenuators are from the idle state.

When the attenuators are in idle state, the two inputs to the directioncomparator should be roughly balanced. This balance may be changed bymodifying the gains of the High Pass Filter sections (HPF) in thecontrol paths.

Referring again to FIG. 8, the diagram shows an example speech signalbeing transmitted in the RX speech path, and the resulting echo waveformin the Tx path due to acoustic coupling from speaker to microphone. Thepeak detector filter follows the envelope of the signal closely with afast decay rate. This fast decay rate is useful so that the speechdetectors will only be triggered momentarily by short deviations inbackground noise. If a noise spike is shorter than a few milliseconds,the peak detector output will decay back to the average noise levelfaster than the attenuators can swing to a full-on state. The"HOLD-OVER" timer is not preset until the attenuator reaches the full-onstate, therefore this makes the switching insensitive to large (butshort) deviations in background noise. Although the attenuators mightstart to deviate on a noise spike, they will return to the correct statemore quickly than prior art algorithms, that have a long exponential orlinear rate of decay for peak detectors.

FIG. 8 also shows the comparator filter output for the Rx path. Thedecay characteristic of this filter, which is of approximately invertedexponential form, prevents the echo waveform in the Tx path from causinga switch to Tx after the Rx signal disappears. The comparator filteroutput in the Tx path is forced to follow the Tx peak detector outputuntil the switching comparator switches to Tx.

The various state constants can be stored in 16 bit registers asfollows:

Tx Speech Detector Threshold

Default value

`01COh`

Address 22h

This register is used by the handsfree program as a threshold level forthe speech detector comparators.

RX Speech Detector Threshold

Default value

`00EOh`

Address 23h

This register is used by the handsfree program as a threshold level forthe speech detector comparators.

IDLE State Register

Default value

`0026h`

Address 24h

This register is used to program the idle state level for handsfreeoperation. Bits (b5 to b0) in this register represent a number referredto as "IDLE" in the formula below.

Idle level (in dB)=(1.5 dB×IDLE)-72 dB.

Comparator Decrement Constant

Default value

`0004`

Address 25h

This register is used to program the decay rate of the comparator lowpass filters.

Ramp-out Timer Register

Default value

`00AO`

Address 26h

This register is an 8 bit binary number used by the handsfree program asa timer present value to program the ramp-out (ramp-down time) of theattenuators when decaying back to idle state.

Decay time per 1.5 dB attenuator step=(timer present value)×(0.5 msec)

Hold-over Timer Register

Default value

`019Oh`

Address 27h

This register is an 8 bit binary number used by the handsfree program asa timer preset value to program the hold-over time of the attenuators.This is the length of time that the attenuator holds full state beforestarting to ramp down to idle state.

Tx High Pass Filter Gain Register

Default value

"035Oh"

Address 28h

This register is used to program the gain of the digital filter in thevoice detector path.

RX High Pass Filter Gain Register

Default value

`035Oh`

Address 29h

This register is used to program the gain of the digital filter in thevoice detector path.

Filter Coefficient A1 Register

Default value

`E024h`

Address 2Ah

This register is used to program the A1 coefficient in the HPF.

Filter Coefficient A2 Register

Default value

`1000h`

Address 2Bh

This register is used to program the A2 coefficient in the HPF.

Filter Coefficient B1 Register

Default value

`19COh`

Address 2Ch

This register is used to program the B1 coefficient in the HPF.

Filter Coefficient B2 Register

Default value

`F540h`

Address 2Dh

This register is used to program the B2 coefficient in the HPF.

The above described circuit has a good tolerance to intermittentbackground noise and does not suffer to the same extent as prior artcircuits from a hollow sound due to band limiting of the speech signals.The use of the special filter design in the directional controlcomparator minimizes switching on echo signals or reverberation.

I claim:
 1. A handsfree telephone circuit with separate transmit andreceive audio paths for connection to a microphone and speakerrespectively, comprising a controllable attenuator in each of saidtransmit and receive paths; attenuator control means responsive to levelcontrol signals for incrementally setting the attenuation level of saidattenuators, said control means responsive to a directional controlsignal for determining which audio path at any moment is a controllingpath having control of said attenuators and which audio path is anon-controlling path; a pair of monitoring circuits for generating saidlevel control signals, said monitoring circuits defining respectivemonitoring paths connected to each of said transmit and receive pathsfor sensing audio signals therein, each said monitoring circuitcomprising a high pass filter for attenuating background noise, arectifier, a peak level detector, and a speech detector; a pair of lowpass filters having a fast attack and slow decay with a transferfunction of approximately inverted exponential form connected to theoutputs of the respective peak level detectors in the two monitoringcircuits; and a switching comparator for comparing the outputs of saidlow pass filters to produce said directional control signal for saidattenuator control means; and means for disabling the low pass filterreceiving the output of the peak detector in the non-controlling audiopath, the output of said low pass filter in the controlling audio pathpreventing the switching comparator from outputting the directionalcontrol signal upon echo signals appearing in the non-controlling path.2. A handsfree telephone circuit as claimed in claim 1, wherein theoutput of one of said low pass filters is fed to said switchingcomparator through an incrementally variable attenuator that tracks theattenuation level of the attenuator in the audio path whose peak leveldetector is connected to said one low pass filter so as to produce ahysteresis effect that gradually reduces the sensitivity of theswitching comparator to a signal in the non-controlling audio path asthe attenuators move toward maximum and minimum gain respectively.
 3. Ahandsfree telephone circuit as claimed in claim 2, wherein said variableattenuator is connected to the low pass filter connected to the peaklevel detector in the monitoring path connected to the receive path. 4.A handsfree telephone circuit as claimed in claim 1 wherein the highpass filter in each monitoring circuit is a second order biquad filter.5. A handsfree telephone circuit as claimed in claim 4, wherein thesecond order biquad filter has the following Z-domain transfer function:

    Y.sub.(z) /X.sub.(z) =G(1+A.sub.1 Z.sup.-1 +A.sub.2 Z.sup.-2)/(1-B.sub.1 Z.sup.-1 -B.sub.2 Z.sup.-2).sup.1

where G is the gain, and A₁, A₂, B₁, B₂ are constants.
 6. A handsfreetelephone circuit as claimed in claim 1, wherein each said attenuatorhas three stable states, which are a maximum gain state, a minimum gainstate, and an idle state of gain intermediate said maximum and minimumgain states; and a plurality of intermediate transient states throughwhich said attenuators pass stepwise during a transition between any ofthe stable states.
 7. A handsfree telephone circuit as claimed in claim1, wherein said attenuator control means causes said attenuator in thecontrolling audio path to ramp rapidly to the maximum gain state andsaid attenuator in the non-controlling audio path to the minimum gainstate when speech is detected in the controlling audio path and to rampmore gradually back to the idle state in the absence of speech in thecontrolling audio path.
 8. A handsfree telephone circuit as claimed inclaim 6, wherein said intermediate states are separated by a fixedattenuation of 1.5 dB.
 9. A handsfree telephone circuit as claimed inclaim 6, further comprising means for presetting the attenuation levelof each of said attenuators in said stable states.
 10. A handsfreetelephone circuit with separate transmit and receive audio paths forconnection to a microphone and speaker respectively, comprising acontrollable attenuator in each of said transmit and receive paths;attenuator control means responsive to level control signals for settingthe attenuation level of said attenuators and directional controlsignals for determining which audio path has control of saidattenuators; pair of monitoring circuits for generating said levelcontrol signals, said monitoring circuits defining respective monitoringpaths connected to each of said transmit and receive paths and forsensing respective audio signals therein, each said monitoring circuitcomprising a high pass filter for attenuating background noise, arectifier, a peak level detector, and a speech detector; and a switchingcomparator for comparing the outputs of the peak level detectors in thetwo monitoring circuits and producing said directional control Signalsfor said attenuator control means; wherein said filter in eachmonitoring path is a second order biquad filter having the followingZ-domain transfer function:

    Y.sub.(z) /X.sub.(z) =G(1+A.sub.1 Z.sup.-1 +A.sub.2 Z.sup.-2)/(1-B.sub.1 Z.sup.-1 -B.sub.2 Z.sup.-2)

where G is the gain, and A₁, A₂, B₁, B₂ are constants, and where G, A₁,A₂, B₁, B₂ have the following approximate values: G=0.2054 A₁ =-1.9911A₂ =+1.0000 B₁ =+1.6067 B₂ =-0.67222.